(NH2)3C3N3, with the scientific name melamine, is a kind of widely-used organic chemical material. It is mainly used to synthesize melamine-formaldehyde resin, manufacture daily utensils, decorative veneer, textile finishing agent, etc., mix with diethyl ether to generate paper treating agent. Moreover, it can also be used as environmentally-friendly high-performance coating cross-linking agent, flame retardant material, etc. The dicyandiamide method is adopted to produce melamine in early technology. The specific production process is: use CaC2 to produce CaCN2 which generates dicyandiamide after hydrolysis and dimerization, and then heat and decompose the dicyandiamide to produce melamine. However, the dicyandiamide method has a high cost due to use of calcium carbide, resulting in poor economic performance of melamine production process.
In order to solve the above-mentioned defects, the dicyandiamide method was replaced by the urea method gradually after 1970s. In the urea method with urea as the raw material, the following chemical reaction is generated at a certain temperature and pressure or under the action of catalyst:6(NH2)2CO→(NH2)3C3N3+6NH3+3CO2 The above-mentioned synthesis process is generally divided into the following three types based on reaction conditions: high-pressure liquid-phase quenching method (7˜100 MPa, 370˜450° C.), low-pressure liquid-phase quenching method (0.6˜1 MPa, 380˜440° C.) and low-pressure gas-phase quenching method (<0.2 MPa, 390° C.). Compared to the processes by high-pressure liquid-phase quenching method and low-pressure liquid-phase quenching method, the said process by low-pressure gas-phase quenching method has such advantages as short flow, fewer equipment, weak medium corrosiveness, less investment and short construction period and hence has attracted broad attention and has been widely applied. As a matter of fact, the low-pressure gas-phase quenching method has achieved relatively rapid development in recent ten years and accounted for about 55% of total global melamine output.
The system for process by low-pressure gas-phase quenching method is shown in FIG. 1. The flow for such process in the prior art is disclosed in U.S. Pat. No. 4,451,271, CN1188761A and CN1493565A, including the following steps and operation parameters:
(a) Carrier gas pre-heating. The process gas from carrier gas compressor with pressure of 0.1˜0.2 MPa is heated to 360˜400° C. by using carrier gas pre-heater and high temperature molten salt. The heated process gas enters fluidized bed reactor as fluidized carrier gas.
(b) Chemical reaction. Urea melt at about 140° C. is pumped into the fluidized bed reactor with upper pressure of 0.05˜0.1 MPa and temperature controlled within 390˜400° C. Under the action of catalyst, about 85˜90 wt % urea reacts chemically to generate melamine and corresponding volume of ammonia and CO2 (i.e., reaction by-products, usually called tail gas in the industry) as per chemical equation. The generated melamine is gaseous and dissolves in carrier gas and tail gas. The catalyst can be porous alumina, monox, titanium oxide or aluminum silicate colloid. The reaction heat is provided by molten salt immersed in the heating tube of catalyst bed.
(c) Cooling of gas generated from reaction. Carrier gas and tail gas with dissolved melamine are exhausted from the top of fluidized bed, enter hot gas cooler tube side, and then get cooled to 310˜320° C. by low-temperature organic heat carrying agent outside the tube. The high-boiling-point by-products (such as melam) in gases are precipitated by crystallization in the gas stream.
(d) Constant-temperature filtering. The gas from the hot gas cooler further flows into the hot gas filter shell side, enters the filter tube under the action of pressure differential, and gets purified with high-boiling-point by-products and catalyst particles intercepted outside by filtering medium. Filter cake attached outside the filter medium is blown down by the blowback gas and falls on the filter bottom and gets discharged regularly. In order to prevent melamine precipitation due to gas cooling in the filter, it is necessary to heat the filter to ensure constant-temperature filtering.
Hot gas cooling and filtering can cause scarring very easily, blocking tubes and equipment. Hence, there are 2 sets of equipment (1 for service and 1 for standby). This is one of the key technical points and difficulties of the low-pressure gas-phase quenching process.
(e) Gas-phase quenching crystallization. Hot gas at about 320° C. from the filter mixes with cold gas at about 140° C. from urea scrubber, and then gets further cooled to 180˜220° C. At last, the melamine is precipitated by crystallization.
(f) Gas-solid cyclone separation. The melamine crystalline powder enters the melamine collector along with the gas stream to finish the gas-solid separation. The separated melamine is compressed out from the discharging equipment at the bottom and then gets delivered to the product packaging system.
(g) Process gas boosting. Process gas with melamine powder separated still contains ammonia, CO2, melamine particles and small amounts of un-reacted reactants, and then enters urea scrubber after boosting by cold gas blower.
(h) Urea scrubbing and cooling. Process gas from the cold gas blower mixes with low-temperature urea melt from urea pump, flows downwards, and gets scrubbed and cooled by urea. Melamine particles and un-reacted reactants in the process gas enter the urea melt and get further cooled to about 140° C. The urea is heated to 136˜140° C. and then the said urea melt is cooled to 127˜130° C. by using cooler outside the scrubber.
(i) Gas-liquid separation. The gas-liquid mixture from the lower part of urea scrubber withstands the separation of urea and process gas by using specifically designed demister. The separated urea enters the kettle for circulation. Gas-liquid separation of urea scrubber is also one of the key technical points and difficulties of the gas-phase quenching process.
(j) Process gas distribution and circulation. Process gas separated by the gas-liquid separator contains crystallization cooling gas, reactor carrier gas and reaction by-product (tail gas). The crystallization cooling gas circulates from the lower part of crystallizer back to the crystallizer, and is used to cool the reaction gases; fluidized carrier gas circulates back to the reactor after boosting by carrier gas compressor and heating by carrier gas pre-heater, and is used as the fluidized carrier gas of catalyst; the tail gas (masses of NH3 and CO2 account for 50% respectively) is delivered to corresponding treating unit and output volume of tail gas can be adjusted automatically through process gas system pressure which is controlled at 0.01˜0.05 MPa in general.
The said process by low-pressure gas-phase quenching method in the prior art still has the following technical defects:
First, the production efficiency per unit volume of the equipment is low. In the prior art, the process by low-pressure gas-phase quenching method is characterized by low operating pressure in melamine reactor, low partial pressure of reactant and slow chemical reaction, resulting in low production efficiency per unit volume of the equipment. For the purpose of higher yields, reactor with larger volume is needed. For example, the equipment with single line annual production capacity of 30,000t shall be provided with a fluidized bed reactor (with diameter more than 8 m) and a crystallizer. However, the design and construction of large-volume melamine equipment face high technical difficulties of equipment design and manufacturing and large investment. Hence, it is hard to achieve higher single line product capacity in the said process. Second, the product's power consumption is high. In the low-pressure gas-phase quenching method, since the equipment needs large amounts of circulating process gas and the pressure ratio is large, it is necessary to provide high-power carrier gas compressor and cold gas blower. Accordingly, the process consumes more power (no less than 1350 kwh for every ton of melamine in general).
Third, the cold gas blower cannot work stably over long period. In the existing process, the cold gas blower is placed behind the melamine collector, with saturated melamine process gas as the working medium. At the same time, the process gas also contains large amounts of melamine powder which has not been collected by cyclone separator. Affected by large centrifugal force of cold gas blower and body heat loss, the melamine can easily get attached to the gas channel and casing of the gas blower. Accordingly, thick melamine crystal scale is generated gradually, resulting in largely reducing the working efficiency and stability of the blower, shortening the continuous running period of the blower and equipment, and increasing the maintenance frequency and cost of the equipment.
Fourth, recycling or utilization cost of melamine tail gas (by-product) is high. In the process by low-pressure gas-phase quenching method, although the tail gas contains no water and can be utilized for many purposes, since the process gas system pressure can only reach 0.01˜0.05 MPa, pressure of tail gas is correspondingly low and can be utilized only after re-boosting of tail gas. Besides, it is necessary to add turbine compressor or gas blower and hence power consumption and hardware investment are high.